1. Field of the Invention
The present invention relates generally to disk drives, and in particular to a head stack assembly including a ground conductive pad for grounding the slider to a gimbal.
2. Description of the Prior Art
The typical hard disk drive includes a head disk assembly (HDA) and a printed circuit board assembly (PCBA) attached to a disk drive base of the HDA. The head disk assembly includes at least one magnetic disk, a spindle motor for rotating the disk, and a head stack assembly (HSA). The spindle motor includes a spindle motor hub that is rotatably attached to the disk drive base. The hub has an outer hub flange that supports a lowermost one of the disks. Additional disks may be stacked and separated with annular disk spacers that are disposed about the hub. The head stack assembly has an actuator assembly having at least one transducer head, typically several, for reading and writing data from and to the disk. The printed circuit board assembly includes a servo control system in the form of a disk controller for generating servo control signals. The head stack assembly is controllably positioned in response to the generated servo control signals from the disk controller. In so doing, the attached heads are moved relative to tracks disposed upon the disk.
The head stack assembly includes an actuator assembly, at least one head gimbal assembly, and a flex circuit cable assembly. A conventional “rotary” or “swing-type” actuator assembly typically includes an actuator having an actuator body. The actuator body is configured to rotate on a pivot assembly between limited positions about an axis of rotation. A coil support extends from one side of the actuator body. A coil is supported by the coil support and is configured to interact with one or more permanent magnets to form a voice coil motor. One or more actuator arms extend from an opposite side of the actuator body.
A head gimbal assembly includes a transducer head, typically a magneto-resistive (“MR”) head, which is distally attached to each of the actuator arms. To facilitate rotational movement of the actuator, the actuator assembly further includes the actuator body that has a bore and a pivot bearing cartridge engaged within the bore. Each magnetic disk includes opposing disk surfaces. Data may be recorded on a single surface or both along data annular regions. As such, the head stack assembly may be pivoted such that each transducer head is disposed adjacent the various data annular regions from adjacent the outer diameter to the inner diameter of each disk.
A typical head gimbal assembly further includes a load beam, a gimbal attached to an end of the load beam, and a slider supported by the gimbal. The transducer head is disposed within the slider. The load beam has a spring function that provides a “gram load” biasing force and a hinge function that permits the head to follow the surface contour of the spinning disk. The load beam has an actuator end that connects to the actuator arm and a gimbal end that connects to the gimbal that carries the head and transmits the gram load biasing force to the slider to “load” the slider against the disk. A rapidly spinning disk develops a laminar airflow above its surface that lifts the slider including the head away from the disk in opposition to the gram load biasing force. The head is said to be “flying” over the disk when in this state.
Conductive traces (copper for example) are laid on a dielectric layer (such as a polyimide) film formed on the head gimbal assembly. The dielectric layer electrically insulates the conductive traces from the gimbal (which may be formed of stainless steel for example). Such technologies are variously named TSA (Trace Suspension Assembly), NSL (No Service Loop), FOS (Flex On Suspension) and the like. These conductive traces interconnect the elements of the transducer head to the drive preamp and the circuits associated therewith. There are typically four conductive traces for the write and read differential pairs of the transducer head. The conductive traces are electrically connected to the transducer head at a trailing end of the slider. Such conductive traces are typically formed upon the dielectric layer through a deposition and/or etching process. The conductive traces include terminal pads which are disposed adjacent the slider. Various electrical connection techniques may be used to connect the terminal pads to the slider, such as gold ball bonding or wire bonding.
The slider may be glued to the gimbal using structural and conductive epoxies. The structural epoxy is used to hold the slider in place. The conductive epoxy (such as silver epoxy) is applied for electrical and thermal conductivity. The conductive epoxy provides a conductive path to electrical ground from the slider to the gimbal which in turn is connected to the load beam, the actuator arm and eventually the disk drive base. Such conductive path is not well controlled. In order to establish a controlled impedance path the conductive epoxy has to electrically breakdown. This involves application of voltage between the slider and the gimbal in excess of the “breakdown voltage”. This can cause significant current flow in close proximity to the transducer head elements which may damage them. In addition, use of conductive epoxies has other problems. The thermal expansion tensor of the silver conductive epoxy has significant variations with temperature due to the presence of silver particles. Further, the silver conductive epoxy may cause fly height variation of the slider due to crown effects.
As disk drives have progressed to higher areal densities the fly height has correspondingly been reduced. The reduction in fly height has made head (slider)-to-disk interactions more likely. In particular, such close proximity of the slider to the disk may result in undesirable electrical discharge between the slider and the disk, as the electrical path between the disk and the slider may have less resistance than the electrical path from the slider to the gimbal through the conductive epoxy. Accordingly, there is a need in the art for a disk drive having an improved head stack assembly design in comparison to the prior art.